1. Field of the Invention
The present invention generally relates to a method of transferring a wafer, and particularly to a method of transferring a wafer to or from a load cup.
2. Description of the Prior Art
Chemical mechanical polishing generally removes material from a semiconductor wafer through a chemical or a combined chemical and mechanical process. In a typical chemical mechanical polishing system, a wafer is held by a polishing head in a feature side down orientation above a polishing surface. The polishing head is lowered to place the substrate in contact with the polishing surface. The wafer and polishing surface are removed relative to one another in a predefined polishing motion. A polishing fluid is typically provided on the polishing surface to drive the chemical portion of the polishing activity. Some polishing fluids may include abrasives to mechanically assist in the removal of material from the wafer.
A wafer transfer mechanism, commonly referred to as a load cup, is utilized to transfer the wafer into the polishing head in a feature side down orientation. As the feature side of the wafer faces the load cup while the wafer is retained therein, care must be taken to avoid damage to the feature side of the wafer through contact with the load cup. For example, the feature side of the wafer may be scratched by surfaces of the load cup that supports the wafer during the transfer process with the polishing head. Additionally, particulates generated during the wafer transfer or generated by contact of the wafer to the load cup may be carried on the wafer's surface to the polishing surface. During polishing, these particulates may cause substrate scratching, which results in non-uniform polishing and device defects. Therefore, it would be advantageous to minimize contact of substrate to load cup.
FIG. 1 is a schematic view of a conventional chemical mechanical polisher. The chemical mechanical polisher 10 includes a base 12, a head clean load/unload (HCLU) station 14, and a rotary bearing 16. The base 12 includes a polishing pad 18 disposed on the base 12. The HCLU station 14 includes a load cup 20 for loading/unloading wafers on/from the polishing head. The rotary bearing 16 includes a plurality of polishing heads 22 to hold and rotate wafers on the polishing pads 18. The load cup 20 includes a pedestal support column 26 to support a pedestal 24. Wafers can be transferred from the pedestal to the polishing head 22 or from the polishing head 22 to the pedestal.
Referring to FIG. 2, a pedestal film 27 may be disposed on the upper surface of the pedestal 24 for contacting the feature side (i.e. the side having IC devices) of the wafer. The spray orifice 28 extends through the pedestal 24 and the pedestal film 27. The bottom surface of the polishing head 22 and the top surface of the pedestal film 27 are washed at the load cup 20 by the ejection of washing fluid through the spray orifice 28. Each wafer is loaded by a transfer robot (not shown) from a loadlock chamber (not shown), onto the load cup 20.
The transfer robot includes a robot blade that is inserted into the loadlock chamber and lifts each wafer individually from the loadlock chamber and places the wafer above the pedestal 24 of the load cup 20. For avoiding the contact of the wafer with the load cup 20, a fluid (such as deionized water) is generally sprayed from a spray orifice (which may be same as or different from the spray orifice 28) extending through the pedestal and the sprayed fluid is between the wafer and the pedestal 24 to float the wafer, such that the contact of the wafer with the load cup is minimized. Thereafter, the polishing head 22 on the rotary bearing 16 holds the wafer away from the pedestal 24 for a subsequent polishing process.
The polished wafer is unloaded from the polishing head 22 and placed into the load cup 20. Similarly, for avoiding the contact of the wafer with the load cup 20, a fluid is sprayed and between the wafer and the pedestal 24 to float the wafer, such that the contact of the wafer with the load cup is minimized. After the load cup 20 is fully filled with the fluid, the surface tension of the fluid may help pulling down the wafer from the polishing head to place the wafer into the load cup. After the wafer is placed into the load cup 20, the wafer may be taken from the load cup by a transfer robot to the next process system.
A conventional technique, such as U.S. patent application publication No. 2005/0274393, which is incorporated herein by reference, discloses a process for cleaning a semiconductor wafer, in which, a cleaning fluid dissolving an ion-forming gas is used to wash polished wafers to reduce or eliminate charge-up damage caused by friction which is generated between the wafer and rinsing water or other fluid as the wafer is rotated during the cleaning process. U.S. Pat. Nos. 6,569,769 and 6,294,470, which are incorporated herein by reference, disclose a chemical mechanical polishing process, in which, an aqueous liquid medium containing a polyelectrolyte is used with polishing slurry to polish wafers, to effectively planarize an oxide layer, even the starting oxide layer has significant topographical variation.
However, the inventors of the present invention found, during a chemical mechanical polishing process, the disappointing yield is partly attributed to a damage caused during the wafer transfer, not the polishing or cleaning process. Therefore, there is still a need for the improvement of wafer transfer.